The invention relates to a process for the preparation of D,L-methionine with a high bulk density, where the methionine is purified by recrystallization.
L-Methionine is an essential amino acid which is of great industrial importance as a feed supplement. Since D- and L-methionine are of identical nutritional value, the racemate is usually used as feed supplement. The synthesis of D,L-methionine proceeds starting from methylmercaptopropionaldehyde and hydrogen cyanide with the preparation of the intermediate 5-(2-methylmercaptoethyl)hydantoin, which can be converted to the methioninate by hydrolysis.
Various processes are known both for the hydrolysis of hydantoin and also for the subsequent release of methionine from its salt. The present invention relates to the preparation of methionine by the so-called potassium carbonate process, which is described for example in EP 1 256 571 A1 and DE 19 06 405 A1. In this connection, 5-(2-methylmercaptoethyl)hydantoin in aqueous solution is firstly reacted with potassium carbonate to give potassium methioninate with the release of carbon dioxide and ammonia. By introducing carbon dioxide, the basic potassium methioninate solution is neutralized and methionine is precipitated out. The crude methionine obtained in this way, however, is produced in the form of platelet-like or flake-like, poorly filterable crystals, which are shown in FIG. 1.
To control the foaming and to improve the crystal quality, the crude methionine precipitation according to EP 1 256 571 A1 takes place in the presence of an antifoam. This process has the disadvantage that methionine is obtained in the form of spherical, but porous particles, which are shown in FIG. 2. Because of its porous structure, the methionine obtained in such a way has to be washed with large amounts of water and dried, incurring high energy costs, in order to arrive at a marketable product.
The addition of additives during the crude methionine precipitation can improve the crystal quality. As additives, for example sorbitan laurate, polyvinyl alcohol, hydroxypropylmethylcellulose, gluten or casein are known from JP 11158140 and JP 10306071. According to these processes, methionine crystals with a bulk density of up to 770 g/l are obtained. It has proven to be disadvantageous for these processes that they are carried out as batch processes or in merely semicontinuous mode.
It is likewise known to improve purity and bulk density of methionine by recrystallization of crude methionine. JP 2004-292324 discloses, for example, the recrystallization of crude methionine by adding polyvinyl alcohol or gluten, giving pure methionine with a bulk density of up to 580 g/l. The recrystallization takes place by the dropwise addition of a hot methionine solution to a cold methionine suspension, with methionine precipitating out as a result of cooling the hot solution. A disadvantage has again proven to be that this process is not carried out continuously.
EP 1 451 139 A1 describes the recrystallization of methionine in the presence of hydroxyethylcellulose, with initially methionine crystals having a bulk density of up to 620 g/l being obtained. In this case, a disadvantage has proven to be that in a continuous recrystallization process there is an accumulation of the continuously added additive as a result of reusing the filtrate for dissolving crude methionine and that an increasing additive concentration results in a reduction in the bulk density. For this reason, hydroxyethylcellulose is not advantageous for use as crystallization additive in a continuous process in which the filtrate of the pure methionine is reused for dissolving crude methionine. The reuse of the recrystallization filtrate is of decisive importance for the economic feasibility of the process on an industrial scale since losses of dissolved methionine are avoided and the generation of wastewater is minimized.
JP 46 019610 B1 describes a process for the recrystallization of methionine, which however does not allow to achieve high bulk densities for methionine.
It is an object of the present invention to provide a process for the preparation of methionine which avoids the described disadvantages. The methionine obtained by the process should be readily filterable and have a high bulk density. Furthermore, the process should be able to be carried out in continuous mode and in particular should avoid the negative consequences of accumulation processes.